1. Field of the Invention
The present invention relates to an apparatus for manufacturing an optical fiber Bragg grating, an optical fiber having a Bragg grating manufactured by the optical fiber Bragg grating manufacturing apparatus, and an optical fiber laser, and more particularly, to an apparatus for manufacturing an optical fiber Bragg grating having a reflection characteristic in a mid-infrared wavelength band of more than 2 μm, an optical fiber having a Bragg grating manufactured by the optical fiber Bragg grating manufacturing apparatus, and an optical fiber laser.
2. Discussion of Related Art
An optical fiber Bragg grating is designed to periodically change a refractive index of an optical fiber core, and it reflects light of a certain wavelength related to a refractive index change cycle and transmits light of other wavelengths. A wavelength reflected in the optical fiber grating is referred to as a Bragg wavelength λB and is defined by Equation 1:
                                          λ            B                    =                                    2              ⁢                                                          ⁢                              n                eff                            ⁢              Λ                        N                          ,                            (                  Equation          ⁢                                          ⁢          1                )            where Λ denotes a cycle of the optical fiber grating at which a core refractive index changes, and neff denotes an effective refractive index of the core. A wavelength of the optical fiber grating is in proportion to an effective refractive index and a cycle of the grating. N denotes an order of the grating which is a natural number. Several Bragg wavelengths exist according to N, but in most of the optical fiber gratings, N=1 is a first-order Bragg grating reflection wavelength. Hereinafter, a Bragg wavelength means a first-order Bragg grating reflection wavelength unless there is a special comment on it.
FIG. 1a shows signal characteristics of a conventional optical fiber Bragg grating, and FIG. 1b shows characteristics of the conventional optical fiber Bragg grating. Referring to FIG. 1a, an optical fiber 10 has an optical Bragg grating (FBG) 11 formed therein. When an incident signal having a wavelength of broadband light is irradiated to the optical fiber Bragg grating 11, a Bragg wavelength λB is reflected as a reflected signal 13, and other wavelengths transmit the optical fiber 10 as a transmitted signal 14. Referring to FIG. 1b, when an input signal 12 is applied to the optical fiber 10, only a wavelength λB1 is reflected, and the remaining signals are transmitted (see (a), (b) and (c) of FIG. 1b).
FIG. 2 is a schematic block diagram of an apparatus for manufacturing the conventional optical fiber Bragg grating. The conventional optical fiber Bragg grating manufacturing apparatus of FIG. 2 comprises a laser beam source 21 and an optical system 22 for changing a core refractive index, a broadband light source 23 and an optical spectrum analyzer (OSA) 24 for observing an optical fiber Bragg grating 26 made in an optical fiber 25.
As the optical fiber 25, a silica optical fiber for an optical communication is usually used, and when a ultraviolet (UV) light from a laser is irradiated to the silica optical fiber, a refractive index of a portion which catches the UV light becomes different from a refractive index of a portion which does not catch the UV light. In this instance, even though light irradiated by a laser is removed, a changed refractive index is maintained “as is”. Thus, if a UV beam from the laser 21 is irradiated to the optical fiber 25 through the optical system 22 for a light intensity change with a cycle of Λ corresponding to a first-order Bragg reflection wavelength λB1, a core refractive index can be periodically changed. For the optical system 22, there is a method for using an interferometer and a method for using a phase mask.
If output of the broadband light source 23 containing a Bragg wavelength is irradiated to the optical fiber 25 and a UV beam from the laser 21 is irradiated to the optical system 22 while observing its output through the optical spectrum analyzer 24, a degree to which a Bragg grating 26 is inscribed on the optical fiber 25 can be adjusted. A signal of the whole wavelength is detected before the UV beam is irradiated. However, when the UV beam is irradiated, light reflected at the Bragg wavelength is generated while the Bragg grating 26 is inscribed, and the amount of reflection is gradually increased as the UV beam irradiating time is increased. Thus, by monitoring a transmitted signal with the Bragg wavelength, if the irradiation of the UV laser beam is stopped when the transmitted signal with the Bragg wavelength reaches an appropriate intensity, it is possible to manufacture an optical fiber Bragg grating having a desired reflectivity.
The optical fiber Bragg grating 26 is used for an optical communication or an optical sensor. Since it is easy to obtain a variety of broadband light sources 23, and a real-time measurement is possible using the optical spectrum analyzer 24, it is convenient to manufacture the optical fiber Bragg grating 26. Recently, there has been a need for an optical fiber Bragg grating for a mid-infrared wavelength band of more than 2 μm whose use is being broadened to fields such as medical science, military purposes, environmental purposes, and space engineering.
However, it is difficult to manufacture an FBG with a Bragg wavelength at the mid-infrared wavelength band as compared to fabrication of an FBG with a Bragg wavelength at an optical communication wavelength band due to lack of measuring equipments at the mid IR wavelength band. In the existing optical fiber Bragg grating manufacturing apparatus, the optical system 22 is tuned to the first-order Bragg grating wavelength λB, and the broadband light source 23 and the optical spectrum analyzer 24 use a wavelength region containing λB1. In order to make an optical fiber grating that has a desired reflectivity, an intensity change of a Bragg wavelength (according to irradiation of a laser beam) must be measurable. The optical spectrum analyzer 24 is best suited for this purpose, but most of the currently used optical spectrum analyzers 24 can only be used in a wavelength of less than 2 μm. Thus, an existing technique for manufacturing the optical fiber Bragg grating using the optical spectrum analyzer 24 has a difficulty in manufacturing the optical fiber Bragg grating 26 in a mid-infrared wavelength band.
Of course, there is a monochromator used as equipment for measuring an intensity change in a wavelength band of more than 2 μm, but it is better to use the optical spectrum analyzer since there are advantages in real-time operation characteristic and convenience of use. However, there has been little research on manufacturing the optical fiber Bragg grating with a Bragg wavelength at mid IR range because there is no appropriate observing equipment.